Diel variations of the attenuation, backscattering and absorption coefficients of four phytoplankton species and comparison with spherical, coated spherical and hexahedral particle optical models

Diel variations of inherent optical properties (absorption coefficient, attenuation coefficient and volume scattering function at 124°) of four species of phytoplankton were measured in the laboratory and were simulated using a homogeneous spherical model, a coated spherical model and a homogeneous hexahedral model. The required inputs to run each optical model were acquired from the measurements; the real and imaginary parts of the refractive index were determined from the intracellular carbon and absorption coefficient, and particle size distributions from the Coulter counter. We conducted a sensitivity analysis on the inherent optical properties in response to changes in the slope of Junge distributions that were used to represent non-phytoplankton particles of radii less than 1.12 µm (the minimum size of the Coulter counter), realistic maximum and minimum values of the refractive indices used for the shell and core, shell thickness, cell radius and the number of cells. We found that the shell's refractive index is the most important factor influencing the backscattering ratio. We found that the coated spherical model reproduced the observed optical properties best for all species possessing a shell. The hexahedral and homogenous spherical models give relatively good results for the absorption and attenuation coefficients; but underestimated the volume scattering function at 124°. Correlations between the measured backscattering cross sections and carbon are significant only for E. huxleyi and D. tertiolecta. In situ measurements will be necessary to determine if our models can reproduce the diel variations of backscattering that are observed in the ocean.